CFD Analysis of Explosions with Hydrogen-Methane-Air Mixtures in Congested Geometries

Hydrogen is an enabler for de-carbonising the energy system in Europe by 2050. In the UK, several projects are investigating the feasibility of gradually blending hydrogen into the natural gas pipelines with the aim to reach 100% hydrogen in the gas network. However, the safe use of hydrogen as a fuel presents different challenges than conventional hydrocarbon-based fuels Advanced consequence models are powerful tools that can be used to support the design process and estimate the consequences of potential accidents. This paper analyses the predictive capabilities of two combustion models for explosion for hydrogen, methane and hydrogen-methane blends. The analysis involves the default combustion model in the commercial version (FLACS-CFD v21.2), and a new combustion model implemented in an in-house development version where the model for premixed turbulent combustion incorporates Markstein number effects (FLACS-CFD v21.2 IH). Experiments performed by Shell in unconfined pipe-racks, some of which were part of the EU funded project NaturalHy, are considered. The simulation results from both versions of FLACS-CFD are within a factor of 2 of the overpressures observed in the experiments. However, FLACS-CFD v21.2 IH appears to give an improved representation of the overpressure trend with variations in the hydrogen equivalence ratio observed in the experiments.publishedVersio

CFD-Model for the Photothermal Conversion Process in Ionic Nanofluids

Harnessing energy from a sustainable source like the Sun may be one of the key solutions for the increasing demand of energy. However, conventional solar harvesters are relatively low efficient so that use of solar energy is challenging. Several experimental studies have shown in the last decades that the optical and thermophysical properties of the working fluid in solar collectors can be enhanced by adding nano-sized particles. These findings have led to the development of the Direct Absorption Collector (DAC), which is a more promising device compared to the most widely used solar collector, i.e. the flat plate collector. However, a reliable theoretical description of the process phenomena is required for the optimization of the design. In this research, photothermal energy conversion in nanofluids was numerically studied using a Computational Fluid Dynamics (CFD) model. A DAC of cylindrical shape with incident light on one of its surfaces was adopted for simulations. The Eulerian two-phase transient model included the volumetric absorption of light, losses to the surroundings and the Brownian motion. The validation of the model with experimental data demonstrated low discrepancies. The model was studied parametrically, altering the extinction coefficient and specific heat of the base fluid, as well as the surface transparency, collector height, solar concentration, particle volume fraction and particle size. The enhancement in efficiency (20%) due to the use of nanofluids was demonstrated by comparison against a selective surface absorption collector. The radiative and convective losses from the DAC surfaces were increased with the nanoparticle volume fraction and with the solar concentration. As the collector height was reduced, the maximum average temperature increased. A maximum temperature of 200.8°C was observed for a 1 cm nanofluid column and 10 sun, where 1 sun equals 1000 W m^(-2). For a 1 cm solar collector and 2.3 sun, a maximum thermal receiver efficiency of 67% was found for 50 ppm. Increasing the particle size did not lead to a significant enhancement in the receiver efficiency. Nevertheless, it resulted in significant particle deposition. A strong dependency on the size of the nanofluid column and convection currents was shown. The efficiency was enhanced by 14.5% for the case when the incident light was to the bottom surface in comparison to the case when the incident light irradiated the top surface. The maximum velocity of the dispersed phase was 0.15 cm s^(-1), which was found for the case when particle concentration was 1.25 ppm. Finally, design recommendations were presented based on the performed theoretical analysis

CFD modelling of hydrogen and hydrogen-methane explosions – Analysis of varying concentration and reduced oxygen atmospheres

This paper evaluates the predictive capabilities of the advanced consequence model FLACS-CFD for deflagrations involving hydrogen. Two modelling approaches are presented: the extensively validated model system originally developed for hydrocarbons included in FLACS-CFD 22.1 and a Markstein number dependent model implemented in the in-house version FLACS-CFD 22.1 IH. The ability of the models to predict the overpressure and the flame arrival time for scenarios with different concentrations of hydrogen, and thus different Lewis and Markstein numbers, is assessed. Furthermore, the effect of adding methane or nitrogen on overpressure for different regimes of premixed combustion are investigated. The validation dataset includes deflagrations in the open or in congested open areas and vented deflagrations in empty or congested enclosures. The overpressure predictions by FLACS-CFD 22.1 IH are found to be more accurate than those obtained with FLACS-CFD 22.1 for scenarios with varying hydrogen concentrations and/or added nitrogen or methane in the mixture. The predictions by FLACS-CFD 22.1 IH for lean hydrogen mixtures are within a factor of 2 of the values observed in the experiments. Further development of the model is needed for more accurate prediction of deflagrations involving rich hydrogen mixtures as well as scenarios with other fuels and/or conditions where the initial pressure or temperature deviate significantly from ambient conditions.publishedVersio